Molded polyurethane articles are used on a large scale in the automobile industry as body parts, such as aprons, spoilers, fenders and the like because of their toughness and wear resistance; they are also suitable as shoe soles, heels and for many other purposes.
The polyurethanes are cured and simultaneously molded in closed, heated molds. The reaction injection molding process, the so-called RIM process, has gained particular importance for this purpose.
More recently, molded polyurea articles have been introduced, wherein the terminal hydroxyl groups of the polyols, used in the manufacture of molded polyurethane articles, are replaced by NH.sub.2 groups, so that amines are formed. These amines which are termed polyol amines or polyoxyalkylene amines, react with the polyisocyanates in the same manner as do the polyols. The products obtained, however, are not polyurethanes, but polyureas. Otherwise, the same process conditions and the same release agents are employed.
A significant problem in the molding procedure is that the molded polyurethane or polyurea parts tend to adhere to the inner walls of the mold. Release agents are known in large numbers to those skilled in the art and entrusted with the production of molded articles from reactive compositions to deal with this problem. Such release agents may be applied on the inner walls of the molds or added as internal release agents to the reaction formulation. The internal release agents may belong to different classes of substances. As internal release agents, metal soaps, such as zinc stearate, esters of higher fatty acids, natural or synthetic oils, waxes or silicones have been used in the past.
Those products are particularly preferred as internal release agents, which make special preparation of the molds superfluous, do not adversely affect the properties of the reacted, molded plastic and, as far as possible, do not cause any change in the surface properties of the molded parts, such as an adverse effect on the paintability or application of other coatings. The internal release agents should be well dispersible in the reaction formulation. However, in order to exert their release effect, they should accumulate at the surface of the molded article.
Examples of internal release agents which are to be used especially for the production of molded polyurethane articles by the RIM process, are organosilicon block copolymers which, aside from siloxane blocks, have polyoxyalkylene blocks. The structure of the compounds evidently plays an important role in determining the release behavior.
For example, it may be inferred from German Offenlegungsschrift 25 43 638 that compounds of the formula ##STR2## are said to be inferior to compounds of the formula ##STR3## the subscripts x in formulas III and IV in each case having an average value of 3 to 45, the subscript y having an average value of 8 to 198, Me representing methyl and (--OR).sub.x representing a polyoxyalkylene polymer or a polyoxyalkylene copolymer, R being composed of ethylene groups or butylene groups or mixtures of ethylene or butylene groups with propylene groups, the ratio of ethylene or butylene groups to propylene groups being selected so that the ratio of carbon atoms to oxygen atoms in the whole of the (--OR) block is 2.0:1 to 2.9:1.
Test methods for determining and comparing the releaseability of different release agents are also given in this Offenlegungsschrift.
Later work shows that certain functional groups can improve the releaseability. Siloxanes with such functional groups are known from U.S. Pat. No. 4,076,695. As functional groups, these siloxanes contain carboxyl groups attached to hydrocarbon groups which, in turn, are connected over an Si--C-- or an SiOC-- bond with the siloxane framework. These release agents with carboxyl groups do not, however, form stable mixtures with polyols which contain amine catalysts and, optionally, tin catalysts. Moreover, the gelling time of the polyurethane system is prolonged by the addition of these compounds.
Because of their reactivity, these compounds cannot be used as a constituent of the isocyanate component.
These disadvantages were largely overcome by using as release agents the organosilicon compounds described in U.S. Pat. No. 4,472,341. These silicones have ##STR4## units, R being a low molecular weight alkyl or aryl group. R.sup.1 is bivalent hydrocarbon group which may contain oxygen or sulfur atoms. R.sup.2 is a low molecular weight alkoxy, aroxy or R.sup.3 O(CHR.sup.4 CH.sub.2 O--).sub.x group, R.sup.3 being hydrogen or a lower alkyl group and R.sup.4 hydrogen or methyl and x having a value of 1 to 50. However, its release behavior is not entirely satisfactory. The same is true for siloxanes with organically bound CONR.sub.2 groups, R representing hydrogen.
A further disadvantage of the known internal release agents is that, in the manufacture of molded polyurethane or polyurea articles with a cellular core, they hinder the development or maintenance of the cellular structure and may even cause collapse of the formed foam. The development of the cellular structure in the interior of the molded article is, however, of importance for obtaining molded articles having defect-free surfaces, as well as with respect to the mechanical properties of the molded article obtained.
Reference is also had to U.S. Pat. No. 4,498,929 which discloses functional siloxane release agents to be used with liquid silicone surfactants. The release agent is preferably used in liquid form.